This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/FI98/00447 which has an International filing date of May 28, 1998 which designated the United States of America.
The present invention relates to chemical industry, especially to petroleum refining. Particularly, the object of the invention is a process for producing high grade middle distillate without substantially altering the distillation range. The product can for instance be used as a diesel fuel.
A low content of sulfur and aromatic compounds, a high cetane number, and an adequate density are among the particular properties of a high grade diesel fuel to be mentioned.
The increasingly strick environmental requirements, in particular regulations limiting the exhaust emissions from the fuels are continuously increasing the demands made on the properties of a high grade fuel. Less polluting diesel fuels are badly needed. Lowering the content of sulfur and aromatic compounds in diesel fuels has an influence on the particle emission from a diesel engine. Further, lowering the amount of aromatic compounds and increasing the cetane number reduce emissions of nitrogen oxides, and a high cetane number seems to reduce the formation of smoke at low temperatures, and particle emissions. In addition, lowering the content of polynuclear aromatic compounds reduces the health hazards associated to diesel exhaust gases. In particular, the emissions from a diesel engine are significant at low temperatures, for instance in wintertime in countries where the temperature remains an extended period of time under 0xc2x0 C., or even less. Such conditions are very demanding for a diesel engine.
The density of a diesel fuel and accordingly the energy content in a unit volume thereof should remain constant throughout the year to ensure the smooth runnig of the engine to reduce emissions therefrom.
Being heavier, the low temperature properties of a diesel fuel are far more important than those of gasoline. In a cold climate such low temperature properties of a diesel fuel should be good. The diesel fuel must remain liquid in all conditions of use, and it may not form precipitates in the fuel feeding devices. The low temperature properties are evaluated by determining the cloud and pour points, as well as the filterability of the fuel. Favourable low temperature properties of a diesel fuel, and a high cetane number are somewhat contradictory. Normal paraffins have high cetane numbers, but poor low temperature properties. On the other hand, aromatics have superior low temperature properties, but low cetane numbers.
Several liquid hydrocarbon fractions contain relatively high amounts of aromatics. Various methods for reducing the content of aromatic compounds and therefore increasing the cetane number are familiar to those skilled in the art. One of these methods is hydrogenation. In hydrogenation the middle distillate is treated with hydrogen at an elevated pressure in the presence of a hydrogenation catalyst. Hereby the cetane number of the diesel fuel increases. In comparison to the feed, the low temperature properties of the fuel are not essentially changed.
On the other hand, there are processes for selectively cracking off normal paraffins that lead to poor properties at low temperatures. In these processes the catalyst used is normally a zeolite with a suitable pore size. Only normal paraffins with straight chains, or paraffins with moderately branched chains can penetrate into the pores. As examples of such zeolites can be mentioned ZSM-5, ZSM-11, ZSM-12, ZSM-23, and ZSM-35, the use thereof being described in U.S. Pat. Nos. 3,894,938, 4,176,050, 4,181,598, 4,222,855, and 4,229,282. With normal paraffins removed, the low temperature properties of the product are improved, but the cetane number is lowered and the content of aromatic compounds is usually increased. Especially heavy feeds are treated with such a process with which waxy components are desired not only to be removed, but also to be converted to other, more valuable materials. Moreover, this process is applicable to lighter middle distillate feeds, as is disclosed in PCT Patent Publication WO95/10578. The said publication relates to a method for converting a hydrocarbon feed containing waxes, and at least 20% by weight thereof boiling above 343xc2x0 C., to a middle distillate product with a lower wax content. According to this method the feed is contacted in the presence of hydrogen with a hydrocracking catalyst containing a carrier, at least one hydrogenation metal component selected from the metals of the group(s) VIB and/or VIII of the periodic table of the elements, and a zeolite with a large pore size, the diameter of the pores being between 0.7 and 1.5 nm, and then the hydrocracked product is contacted in the presence of hydrogen with a catalyst for wax removal containing a crystalline molecular sieve with a medium pore size selected from metallosilicates and silicoaluminophosphates. The method comprises both a hydrocracking step and a step for wax removal using respectively a different catalyst.
U.S. Pat. No. 5,149,421 discloses a process for isomerizing a lubricating oil with a catalyst combination containing a silicoaluminophosphate molecular sieve as well as a zeolite catalyst. Further, U.S. Pat. No. 4,689,138 describes a method for wax removal from lubricating oils and from middle distillates. The hydrogenation of aromatic compounds is not discussed in this patent. The catalyst was a SAPO-11 to which the hydrogenating metal was added in an unusual way, namely directly to the crystallization solution of the molecular sieve.
In U.S. Pat. No. 4,859,311 wax is removed from a hydrocarbon feed boiling above 177xc2x0 C., hereby converting the hydrocarbons at least partially and selectively to non-waxy hydrocarbons with a lower molecular weight. Essentially, also this patent relates to the production of a lubricating oil.
Moreover, there are processes for removing wax from distillates used as starting feed materials, by isomerizing, the waxy paraffins without any substantial cracking, such as described in the patent FI 72 435. Here, the typical feed materials are hydrocarbons boiling above 180xc2x0 C. ( greater than C10). Hereby the low temperature properties of the product are improved in comparison with the feed.
Wax removal is also carried out using, methods in which heavy normal paraffins are removed with a solvent to improve the low temperature properties of the product. Surprisingly, it has now been found that it is possible to produce, by using a single treatment and middle distillates as the feed, a high grade diesel component with superior low temperature properties and a low content of aromatic compounds, without significantly changing the cetane number of the product. An optimal balance between the cetane number, the content of aromatic compounds and the low temperature properties is attained in the diesel fuel by treating these distillates in a specific way.
Accordingly, one object of the present invention is a process for producing from a middle distillate a high grade diesel fuel with superior low temperature properties and a low content of aromatic compounds. Another object of the invention is to provide a process for producing diesel fuel that leaves the cetane number of the product essentially unchanged even though normal paraffins are isomerized to isoparaffins with lower cetane numbers. The cetane features lost with the isomerization of the paraffins are recovered by hydrogenating the aromatics. In addition, the treatment can cause opening of ring structures and minor cracking. Due to this cracking the product may also comprise lighter isopraffins than the feed, these lighter isoparaffins having superior low temperature properties as well as high cetane numbers.
The present invention relates to a process for producing from a hydrocarbon feed as the starting material, especially from a middle distillate a product suitable as a diesel fuel with improved low temperature properties and a low content of aromatic compounds.
The invention is characterized in that the feed material is contacted in a single reaction step, in the presence of hydrogen, and at an elevated temperature and pressure, with a bifunctional catalyst containing a hydrogenating metal component in addition to a molecular sieve and a carrier. The catalyst ensures the removal of aromatics and the simultaneous isomerization of paraffins.
A suitable isomerizing component in the method of this invention is a molecular sieve, used in an amount of 20-90 wt-%, preferably 65-80 wt-%, relative to the total weight of the catalyst. For instance, a crystalline aluminosilicate, or a silicoaluminophosphate may be used as a molecular sieve.
The method of the invention provides a diesel fuel having a very low total content of aromatics as well as a very low total content of substances consisting of polynuclear aromatic compounds extremely hazardous to health. The use of the diesel fuel according to the invention gives rise to very low levels of emissions detrimental to the environment, comprising for instance sulfur, nitrogen oxides and particles, and to a very weak formation of smoke at low temperatures. The fuel contains very little, if any, sulfur. The process being versatile concerning the feed, the end point of the distillation of the diesel fuel product may be adjusted to a suitably heavy range without adversely affecting the low temperature properties of the product. Further, the seasonal variation of the density and viscosity of a diesel fuel and thus environmental impact of exhaust emissions are reduced.
The feed used according to the invention is a middle distillate. By middle distillate is understood a mixture of hydrocarbons boiling in the range of 150 to 400xc2x0 C. Accordingly, as examples of useful starting feed materials may be mentioned solvents, petrols, as well as light and heavy gas oils. The middle distillate may be for example distillated from such materials as crude oil, or the products of a catalytic cracking or hydrocracking. Concerning the hydrocarbon stream fed to the aromatics removal and simultaneous isomerization step according to the invention, the sulfur content thereof should be below 1000 ppm, and the nitrogen content less than 100 ppm. Preferably, the sulfur concentration is less than 100 ppm and the nitrogen concentration is less than 10 ppm.
According to the invention, the aromatics removal and the simultaneous isomerizing treatment of the middle distillate is accomplished in the presence of hydrogen and a catalyst, at an elevated temperature and pressure. The reaction temperature may vary between 250 and 500xc2x0 C., the pressure being at least 10 bar, the hydrogen feed being at least 100 Nl/l, and the liquid hourly space velocity (LHSV) being between 0.5 and 10 hxe2x88x921. The following conditions are preferable:
LHSV 0.5-3 hxe2x88x921 temperature 300-400xc2x0 C., pressure 50-80 bar and hydrogen flow 200-500 Nl/l.
In the process of the invention the catalyst may comprise any commercial catalyst for wax removal. The essential component of a catalyst for wax removal is a crystalline molecular sieve with a medium pore size. The molecular sieve may be selected from zeolites and silicoaluminophosphates. Useful zeolites include xcex2-zeolite, and zeolites ZSM-11, ZSM-22, ZSM-23, and ZSM-35. The said zeolites are used for instance in the following patents relating to wax removal: FI 72 435, U.S. Pat. No. 4,428,865 and European Patent Publication Nos. 0,378,887 and 0,155,822.
Useful silicoaluminophosphates include SAPO-11, SAPO-31, SAPO-34, SAPO-40, and SAPO-41 that may be synthetized according to the patent U.S. Pat. No. 4,440,871. These silicoaluminophosphates were used as isomerization catalysts in such publications as U.S. Pat. No. 4,689,138, U.S. Pat. No. 4,960,504, and WO 95/10578.
In addition, the catalyst of the invention comprises one or more metal(s) as a hydrogenation/dehydrogenation component. These metals typically belong to the group VIb, or VIII of the periodic table of the elements. Preferably, the metal used is platinum, the amount thereof being 0.01-10 wt-%, preferably 0.1-5 wt-%.
Further, the catalyst comprises as a carrier an inorganic oxide. Known carrier materials include the oxides of aluminium and silicon, as well as mixtures thereof. The relative amounts of the molecular sieve and the carrier may vary widely. The proportion of the molecular sieve in the catalyst is usually between 20 and 90 wt-%. Preferably, the catalyst mixture contains the molecular sieve in an amount of 65-80 wt-%.
If desired, the middle distillate used as the feed may be hydrogenated to reduce the content of sulfur and nitrogen compounds thereof to a suitable level. Any known technology for lowering the sulfur and nitrogen content of a middle distillate may be used as the procedure for sulfur and nitrogen removal. Hydrogenation under hydrogen pressure and by means of a catalyst is normally used to this end to convert the organic sulfur and nitrogen compounds respectively to hydrogen sulfide and ammonia.
The treatment for sulfur and nitrogen removal may optionally be carried out in view of a more advantageous product distribution and an extended operation time.
Any commercially available CoMo and/or NiMo catalyst may be used as the catalyst for sulfur and nitrogen removal. Usually, although not necessarily, the catalyst is pre-sulfided to improve the activity thereof. Without such a pre-sulfiding treatment the initial activity for desulfurization of the catalyst is low. Any process conditions generally known for sulfur removal may be used, such as:
LHSV 0.5-20 hxe2x88x921, temperature 250-450xc2x0 C., pressure  greater than 10 bar, hydrogen flow  greater than 100 Nl/l.
The following conditions are preferable:
LHSV 1.0-5.0 hxe2x88x921, temperature 300-400xc2x0 C., pressure 30-50 bar, hydrogen flow 150-300 Nl/l.
From this desulfurization step the product, free from hydrogen sulfide, ammonia, as well as lighter hydrocarbons, is fed to the step for isomerization and simultaneous removal of aromatics according to the present invention.
The bifunctional catalyst for isomerization and wax removal has an acid function, as well as a hydrogenating function ideally in a good balance with one another. For instance, zeolite catalysts are generally modified by removing aluminium from the crystalline structure, such as by extracting with hydrochloric acid as described in the patent publication EP 0,095,303, or using a water vapor treatment according to the patent publication WO 95/28459, to reduce the acidity, and thus the amount of any unselective reactions.
In the isomerization of the paraffins of the middle distillates the cracking thereof to gasoline and gaseous products is an undesirable reaction to be limited. This may not only be achieved with a known technique by reducing acidic sites in the catalyst, but also, according to our observation, by controlling the nitrogen content of the feed. An excessive nitrogen content lowers the activity of the catalyst, and thus the removal thereof to a certain level is desirable. On the other hand, a completely nitrogen free feed is not always preferable, since the catalyst might then be too acidic. By controlling the nitrogen content of the feed to the isomerization the product distribution may be adjusted to produce the desired diesel component at as high levels as possible, and to improve the selectivity of the isomerization. Preferably, the control is carried out by using organic nitrogen compounds that decompose in the isomerization conditions to form ammonia. This ammonia passivates the acidity of the catalyst, leading to the desired result. The passivation required by various kinds of zeolites and molecular sieves, respectively, is of course different. For instance, with the SAPO molecular sieves the passivation may be expected to be less significant that with zeolites in general. The passivation is not needed if the nitrogen content of the feed is sufficiently high.
The passivation may be carried out by using ammonia, as well as organic nitrogen compounds, preferably aliphatic amines. For instance, tributyl amine (TBA) is preferable since it decomposes easily to form the ammonia needed. The correct nitrogen content of the feed may also be achieved by controlling the degree of the nitrogen removal before the isomerization.
The diesel fuel provided by the process of the present invention is free of sulfur, or contains very low levels thereof, thus being ecologically very acceptable. Further, it is particularly suitable to the demanding low temperature conditions. Since the process is versatile in view of the feed, the end point of the distillation of the diesel fuel product may be adjusted to a suitably heavy range without adversely affecting the low temperature properties thereof. Further. the seasonal variations of the density and viscosity of the diesel fuel, and thus the polluting impact on the environment of exhaust emissions therefrom are reduced.
This combined method for isomerization and simultaneous aromatics removal produces as a by-product low levels of lighter hydrocarbons that may be removed from the diesel product stream by distillation, and conducted further to an optional processing.